Make short uuid specification for allow-discards work

1) strstr " $discarduuids " did not remove the optional
'luks-' prefix from the argument as the documentation says.

2) The lookup seems backwards. $luksdev ($luks in the other code copy)
is the full uuid and thus one should check whether the user-supplied
argument (short form) is contained therein, not the other way around.

Before this commit, the only way to trigger allow-discards was to
specify the full uuid without the 'luks-' prefix.

modules.d/90crypt/crypt-run-generator.sh

@@ -14,7 +14,7 @@ fi
 # parse for allow-discards
 if strstr "$(cryptsetup --help)" "allow-discards"; then
     if discarduuids=$(getargs "rd.luks.allow-discards"); then
-        if strstr " $discarduuids " " ${luks##luks-}"; then
+        if strstr " ${luksdev##luks-} " " ${discarduuids##luks-}"; then
 	    allowdiscards="allow-discards"
 	fi
     elif getargbool rd.luks.allow-discards; then

modules.d/90crypt/cryptroot-ask.sh

@@ -97,7 +97,7 @@ done
 # parse for allow-discards
 if strstr "$(cryptsetup --help)" "allow-discards"; then
     if discarduuids=$(getargs "rd.luks.allow-discards"); then
-        if strstr " $discarduuids " " ${luksdev##luks-}"; then
+        if strstr " ${luksdev##luks-} " " ${discarduuids##luks-}"; then
             allowdiscards="--allow-discards"
         fi
     elif getargbool rd.luks.allow-discards; then